27.2.2 Copying Existing Permissions

You can base a file's permissions on its existing permissions. To do
this, instead of using a series of ‘r’, ‘w’, or ‘x’
letters after the
operator, you use the letter ‘u’, ‘g’, or ‘o’. For
example, the mode

o+g

adds the permissions for users who are in a file's group to the
permissions that other users have for the file. Thus, if the file
started out as mode 664 (‘rw-rw-r--’), the above mode would change
it to mode 666 (‘rw-rw-rw-’). If the file had started out as mode
741 (‘rwxr----x’), the above mode would change it to mode 745
(‘rwxr--r-x’). The ‘-’ and ‘=’ operations work
analogously.